assemble_cvd: Skip rebuilding OS composite disk on snapshot restore

[Elie-Kh]

When restoring from a snapshot, the base images and instance disks have already been restored from the snapshot directory by RestoreHostFiles(). Checking filesystem timestamps to determine whether to rebuild composite disks during snapshot restore can lead to creating_os_disk becoming true, which causes assemble_cvd to abort with 'Restoring from snapshot requires not creating OS disks'.

Only evaluate WillRebuildCompositeDisk() and set creating_os_disk when snapshot_path is empty.

Bug: 554545294

[Databean]

There are build failures reported on this change:

ERROR: /mnt/build/base/cvd/cuttlefish/host/commands/assemble_cvd/BUILD.bazel:24:13: Compiling cuttlefish/host/commands/assemble_cvd/assemble_cvd.cc failed: (Exit 1): cc_wrapper.sh failed: error executing CppCompile command (from target //cuttlefish/host/commands/assemble_cvd:assemble_cvd) external/toolchains_llvm++llvm+llvm_toolchain/bin/cc_wrapper.sh -U_FORTIFY_SOURCE '--target=x86_64-unknown-linux-gnu' -U_FORTIFY_SOURCE -fstack-protector -fno-omit-frame-pointer -fcolor-diagnostics ... (remaining 304 arguments skipped)
  cuttlefish/host/commands/assemble_cvd/assemble_cvd.cc:444:11: error: non-object type 'char *(const char *, int) noexcept(true)' is not assignable
    444 |     index = 0;
        |     ~~~~~ ^
  cuttlefish/host/commands/assemble_cvd/assemble_cvd.cc:447:44: error: no matching member function for call to 'ForIndex'
    447 |       AndroidBuild& build = android_builds.ForIndex(index);
        |                             ~~~~~~~~~~~~~~~^~~~~~~~
  ./cuttlefish/host/commands/assemble_cvd/android_build/android_builds.h:35:17: note: candidate function not viable: no known conversion from 'char *(const char *, int) noexcept(true)' to 'size_t' (aka 'unsigned long') for 1st argument
     35 |   AndroidBuild& ForIndex(size_t index);
        |                 ^        ~~~~~~~~~~~~
  ./cuttlefish/host/commands/assemble_cvd/android_build/android_builds.h:36:23: note: candidate function not viable: no known conversion from 'char *(const char *, int) noexcept(true)' to 'size_t' (aka 'unsigned long') for 1st argument
     36 |   const AndroidBuild& ForIndex(size_t index) const;
        |                       ^        ~~~~~~~~~~~~
  cuttlefish/host/commands/assemble_cvd/assemble_cvd.cc:449:12: error: cannot increment value of type 'char *(const char *, int) noexcept(true)'
    449 |       index++;
        |       ~~~~~^

I was able to reproduce it locally by fetching the change. You can run bazel test //... in the base/cvd directory to build everything and run the unit tests and linters.

[Databean]

Failing on a clang-format validation test

INFO: Build completed, 1 test FAILED, 45435 total actions
  //cuttlefish/host/commands/assemble_cvd:assemble_cvd_format_test_C++_with_clang-format FAILED in 0.2s

[Elie-Kh]

Done, PTAL